The invention relates to a method for checking the signal path of a measuring system, of the type having a collector for charged particles and an amplifier connected to its output for the formation of a voltage or current signal, in which a known current is supplied to the charge collector. The present invention furthermore relates to an apparatus for practicing this method as well as to the application of this method in mass spectrometers, ionization vacuum meters and the like.
Measuring systems which have a charge collector and a measuring amplifier connected thereto, such as those used, for example, in mass spectrometers, ionization vacuum meters and the like, must normally be highly sensitive. This results from the need to record ever smaller particle currents impinging upon the charge collector. If, for example, it is desired to register a particle current of 10.sup.-16 A, which corresponds to a current of about 600 ions or electrons per second, then the resistance that serves to produce a voltage signal in the measuring amplifier must be extremely high. This high sensitivity of such systems requires that the charge collector as well as the first stage of the following amplifier be perfectly shielded. Consequently, this part of the system, which is important for the measurement signal, is usually quite inaccessible.
If it is desired to verify the operation of the signal path in the electronic circuits connected to the output of the charge collector, a known current must be fed into the charge collector. The introduction of a current in the range of 10.sup.-12 A and less, however, constitutes a source of error and inaccuracy, not only in the test, but also in the measuring operation itself. Consequently, sensitivity tests with currents of the above-stated order of magnitude have not been possible heretofore, since none of the former test methods could do without a galvanic connection to the input of the measurement amplifier.
The present invention is addressed to the problem of checking the electronic devices connected to the output of a charge collector, even in the case of very small measurement currents, without thereby negatively affecting the characteristics in the measurement operation itself.